In the wrist, elbow, and other joints of robot arms, as well as in the finger joints of robot hands and the like, actuators are installed for causing a member attached to a shaft in these joints to revolve or rotate within a finite angular range. An actuator configured so that a motor and a reducer are connected concentrically and in series, and that a joint shaft disposed concentrically with the central axis of the motor and reducer rotates around the central axis line, is known as such an actuator, as disclosed in Document 1. Also known is an actuator configured so that a joint shaft perpendicular to the motor shaft is rotated by the output rotation of the motor via a reduction gear mechanism comprising a pair of bevel gears, as disclosed in Document 2.
A sensing mechanism configured so that a reflective plate formed on the outer face of the output shaft of a reducer is detected by a reflecting photosensor disposed outside of the reflective plate is known as a sensing mechanism for detecting the rotational position and the revolving position of a joint shaft, as disclosed in Patent Document 1. A sensing mechanism configured so that a scale is formed on the outer face of a cylindrical rotor attached concentrically to the rotation axis, and that the scale is detected by a reflecting photosensor disposed on the outer side of the scale is also known, as disclosed in Document 3.